Unlike a conventional multi-engine jet, the multiple jet engines of the "overbalanced" or "3x" jet engine configuration of this invention are centerline mounted. Moreover, while each engine according to the configuration of this invention is a "main" engine, capable of safely operating the aircraft in the event of a failure of the other engine(s), the engines are overbalanced in the sense that at least one of the main engines has substantially greater thrust than the other main engine(s).
Indeed, according to a preferred embodiment of this invention, there will be a single main engine having a thrust so far overbalanced in comparison to the thrust of the other main engine(s) that (with reference to the main engines of the corresponding conventional configuration) a single overbalanced engine of this invention will have a thrust equal to the combined thrust of all the engines in the conventional configuration.
The relative thrust of the jet engines of this invention may be understood in comparison to a conventional multi-engine jet as follows, where "x" or "1x" is used to signify a thrust adequate to operate the aircraft at its design cruise speed, and, thus signifies a true multi-engine capacity. A conventional twin multi-engine jet aircraft is generally designed so that each of its two engines has a thrust equal to 1x, and a combined or total thrust equal to 2x. As a "true" multi-engine aircraft (as contemplated by such authorities as, for example, the United States Federal Aviation Administration and its regulations contained in 14 C.F.R. Part 25) it may be said that each of the 1x engines in a conventional twin jet is a true main engine.
In contrast, a preferred embodiment of a two engine configuration according to the present invention will also have two main engines, but one of them will have a thrust equal to the combined thrust of both engines of the conventional twin configuration. That is, according to the present invention, one engine will have 1x of thrust, and the other engine will have 2x of thrust. It follows that the total thrust of the two engines combined will be 3x. Thus, a preferred two engined version of this invention will be a true multi-engine aircraft in which each of the two engines is a true main engine, but in which a single one of the engines has thrust equal to the combined thrust of the comparable conventional twin, and in which the total combined thrust available substantially exceeds the combined thrust of the conventional twin.
According to the preferred method of using the configuration of this invention, it is intended that all of the main engines operate at critical periods of flight, but that only one of the main engines operate at other periods. In use, therefore, the aircraft of this invention is operated at certain phases of flight as a single engined aircraft (most often using the main engine with the significantly higher thrust) and yet having all of the thrust of the conventional multi; and it is operated at other phases of flight as a multi-engined aircraft having even more thrust than the conventional multi. The configuration of this invention realizes a combination of economy, performance and safety having unexpected advantages over either conventional single engine or conventional multi-engine configurations.
Economy, performance and safety are well known considerations in the aircraft industry, and the configuration of this invention will be discussed against this background. To be viable, an aircraft innovation should offer some advantageous mix of performance and safety enhancements at a reasonable economic price.
Important economic considerations include the initial cost and the operating costs of the aircraft. Included within an aircraft's operating costs are such components as fuel, overhaul and other maintenance costs. Those components can be compared against aircraft in different configurations to estimate the projected economies of each.
Performance of different configurations of an airframe can be compared by holding the airframes constant while varying only the engines and the engine configuration, comparing the rated thrust of the engines in one configuration against the rated thrust of the engines in another configuration. Various discounting or weighting elements may be factored into the performance account because of incidental changes in drag or weight or other matters associated with the different engine configurations or, for more basic or conservative calculations, a configuration that is expected to be "cleaner" can nonetheless be assumed to have no less drag than the airframe against which it is being compared (and any advantages in drag can be used to offset potential disadvantages in weight, and both factors may be omitted from the calculations).
Safety can be considered from several perspectives. A basic consideration is related to the multi-engine ratings and requirements codified in such places as part 25 of the regulations promulgated by the United States Federal Aviation Administration (14 C.F.R. Part 25 "Airworthiness Standards: Transport Category").
Drawing from such standards as those, a first safety concern is simply that a "true" multi-engine configuration must allow for redundant engine thrust--in the event of an "engine-out" condition, the other remaining engine(s) must be able to meet all relevant requirements of FAR part 25, or other relevant standard without the power from the failed engine. In the course of the following discussion, "true multi-engine" or "true twin-engine" will refer to a configuration in which all relevant flight standards are meet even with one engine totally shut down.
Likewise drawing from such standards as those, but not limited to considerations of true multi-engine capability are other common safety margin ratings and concerns. A common safety measure is the so-called "balanced field length." This is a length calculated from stop to stop at full take-off gross weight, with an engine failure intervening. The exercise requires that, starting at one end of a runway at a full stop, an aircraft then (a) accelerate to rotation speed (rotation occurs when the aircraft rotates, for example, on its landing gear so as to take-off), (b) simulate an engine failure (so as to require an abort), (c) pause 2 seconds (to simulate the pilot's reaction time to the abort condition), and (d) come to a full stop at the other end of the runway (throttle retarded, full brakes). As used herein, the "balanced field length" corresponds generally to the "accelerate-stop distance" described by the United States Federal Aviation Administration in 14 C.F.R. Part 25, .sctn.25.109 (1993).
The "balanced field length" yields a number, say, 3,000 feet, which tells a pilot how much runway length he or she needs. The balanced field length is calculated at "normal" conditions, including a standard temperature at sea level. If all runways were well in excess of a particular airframe's balanced field length, this factor would become negligible, but even where an airfield is longer than a particular airframe's balanced field length at standard conditions, the balanced field length is known to vary as ambient conditions change.
It is well known, and charts are available to calculate the variations, that the balanced field length is sensitive to many conditions. Perhaps the two most important factors are temperature and altitude. The higher and hotter the ambient conditions, the longer the balanced field length becomes. That is, an airframe's balanced field length might be 3,080 feet standard, but 5,690 feet at 86.degree. F. and at 5,000 feet altitude (higher and hotter than standard) as, for example, may occur at Denver, Colo., and many other airfields.
The balanced field length, as it changes based upon ambient conditions at any given runway is of vital concern to most pilots. For business jet aviation, it may mean that certain flight plans simply must be rerouted or canceled or altered. If, for example an aircraft has a balanced field length under relevant ambient conditions of 5,690 feet, it simply cannot take-off safely at full weight from a runway of 3,500 feet, and a potentially short stage of a multi-stage flight that might have been routed through a 3,500 foot runway stop-over would have to be rerouted to a longer field. Alternatively, the pilot might have to create a shorter balanced field length by decreasing the aircraft's gross takeoff weight, perhaps offloading fuel, and then adding an intermediate stop-over at another airport to refuel.
It can be readily understood that an airframe's safety factor increases as its balanced field length decreases. In order to decrease the balanced field length, and given that only minimal savings can be reasonably attained in several of the stages involved in determining a balanced field length (abort, 2 second delay, throttle retard, and brake), it should be apparent that most savings will come from decreasing the length of the first stage (from stop to rotation). A decrease in the length required to go from a dead stop to rotation will most easily come from an increase in power. Thus, adding thrust will, other things being equal, decrease the balanced field length and increase the safety margin of an airframe.
Finally, and in addition to the safety factors having to do with true multi-engine capability, and a favorable balanced field length (that is, favorable in relation to the actual runways that may be required or preferred for use by an airframe under varying ambient conditions), a third safety factor may be referred to simply as critical stage margin. Dividing any flight into the components: (1) take-off and climb, (2) cruise, (3) descent, and (4) landing, it may be readily agreed that take-off and climb, and landing are critical stages of any flight.
It is not too great an oversimplification to say that a common wish of all pilots is to be flying an aircraft that has available an excess of power, particularly at the critical stages of flight. Despite the multiplicity of specific recovery techniques in the face of different emergencies in the critical stages, it is well known that having a great deal of power/thrust at the pilot's disposal would be a preferred starting point before the trouble occurs.
With the foregoing measures of economy, performance and safety in mind, it should be readily apparent that a single engine aircraft is likely to more economical than a comparable multi-engine aircraft. If the airframe is designed for 3,800 pounds of thrust, it is probably more economical to purchase, overhaul, fuel and maintain a single 3,800 pound engine than a pair of 1,900 pound engines. Likewise (and under the assumption that the single has all the power of the combined thrust of the multi-engines against which it is being compared) a single 3,800 pound engine may be the better performer because it may produce less drag than two 1,900 pound engines, considering their mountings. On the other hand, a conventional single engine aircraft lacks the inherent safety advantages of a true multi-engine aircraft.
Accordingly, it should not be surprising (even without considering the incentives of any legal or regulatory climate that may be created by relevant governmental bodies in terms of true multi-engine ratings and requirements) that commercial aircraft powered by jet engines are almost universally multi-engined. Although it is generally agreed that a single-engine jet aircraft of equal power to a comparable multi would probably be a superior performing design because of the inherent advantages of a single engine relative to multiple engines, and although it is generally acknowledged that such a single-engine jet could be readily designed using today's materials and know-how, there has been no significant commercial use of a single-engine design.
The problem is that the economy (and potential performance) advantages of a powerful single jet engine comes at a cost in terms of safety. A true multi-engine configuration provides for "engine out" backup performance and thus creates a margin of safety not possible with a single engine configuration.
It has long been supposed that the safety advantages of a true multi-engine configuration could never be combined with the economic and performance characteristics of a powerful single-engine configuration. For safety reasons, therefore, the conventional multi-engine configuration has been favored in business aviation at the expense of the efficiency and performance advantages that are might be attained with a single-engine configuration.
It is a specific goal of the configuration of this invention to combine favorable economies and performance with yet further enhanced safety features. The aircraft configuration of this invention attains efficiency and performance advantages of the kind associated with a powerful single engine, while also retaining the safety advantage of a true multi-engine configuration. These results are obtained by employing a centerline mounted, radically overbalanced multi-engine configuration so that the engines are functionally distinct, and by operating the overbalanced engines in a manner to take advantage of that functional distinction.
According to the configuration of this invention embodied in, for example, a two-engine mode, the two engines are of radically unequal thrust and are centerline mounted. But, while the engines are of unequal thrust, each of them is a true main engine in the sense that each of them can meet all "engine out" requirements for true multi-engine ratings. Finally, while there are two engines, they are run in various combinations of one only and both together so that the aircraft is frequently operating on only one or the other of its two engines.
The key concepts of this invention include these: the engines are centerline mounted; all engines of this invention have thrust of at least 1x (and are true main engines in a true multi-engine configuration); at least one of the main engines has a thrust greater than 1x; and the total thrust of "N" engines is greater than Nx. For ease of discussion in connection with certain calculated data presented later in this description, "3x" will designate a hypothetical two-engined jet according to this invention where the thrust of one main engine is equal to 1x; the thrust of the other main engine is approximately 2x; and the total power available with both engines running is approximately 3x. This invention is, however, not limited to that hypothetical embodiment.
The inventor is aware of no other airframe that has ever employed such an overbalanced multiple main centerline mounted jet engine configuration as that of this invention.
U.S. Pat. No. 4,089,493 of Paulson shows an aircraft with two in-line engines, but is not a true multi-engine aircraft and does not suggest the concept of multiple jet engines, both of which are main engines. The engines of Paulson include only one main (turboprop) engine, with a secondary (turbojet) engine. There is no suggestion that the secondary engine of Paulson is a main engine. That is, the secondary engine of Paulson is less than 1x, and, most importantly, the aircraft of Paulson is not a true multi-engine aircraft within the meaning of FAR part 25, as previously discussed herein.
U.S. Pat. Nos. 2,244,763 of Bugatti and 2,978,208 of Halsmer show aircraft with two propellers in line powered by two separate engines. Other examples of aircraft with two propellers mounted in line are shown in U.S. Pat. Nos. 1,850,066 of Altieri; 1,851,857 of Marney; and 2,540,991 of Price. Examples of aircraft having two engines mounted in-line and driving two separate blowers or propellers include U.S. Pat. Nos. 1,132,368 of Lorenc and Lorenc; 2,406,625 of Oglesby; and 2,523,938 of Berliner.
U.S. Pat. No. 4,684,081 of Cronin discloses an auxiliary power and emergency system using a free turbine, and U.S. Pat. No. 3,678,690 of Sohet et al. discloses a convertible composite engine with two engines concentric about a single centerline.
Thus, despite all the work previously done in airframe configurations, there remains a need for a jet engine configuration that can realize the economies and performance of a powerful single-engine jet while enhancing the safety factors typically associated with a true multi-engine configuration. It is a specific object of the overbalanced multiple main centerline mounted jet engine configuration of this invention to provide those benefits of economy, performance and enhanced safety.